Earlier today, I spoke to NPR’s Diane Rehm about jellies and the impact that jellyfish population increases and expansion of some species’ geographic ranges are having on the health of our oceans.

With Bill Dennison and Diane Rehm at NPR.

Jellyfish first appeared around 560 million years ago (long before the time of dinosaurs). They’re 95 percent water, have no brains and no bones and no heart or blood, yet these gelatinous animals are among the worlds’ most resilient organisms.

The jellies simple body plan has remained relatively unchanged. But lately some scientists are concerned the animals are thriving too well – overrunning marine ecosystems, forcing nuclear power plants to shut down and filling the nets of commercial fisherman and shutting down fisheries around the world.

There are both ecological human-related issues causing an “explosion” of jellyfish populations around the world.

Over the years, climate change has raised the average temperature of the ocean. While this rise has negatively impacted organisms like coral, warm season jellies start breeding earlier and have longer active seasons.

Human activities, such as the over-fertilization of our lawns and farms, results in a runoff of excesses of nutrients into the Chesapeake Bay and ocean. This results in what’s known as an algal bloom. When the algae absorbs all of this nutrient run-off, it dies. Bacteria then feeds on the dead algae and removes all dissolved oxygen from the water – this process is called eutrophication and produces “dead zones.” Fish and crabs perish in these dead zones, but not the resilient jelly. Jellyfish can survive with low oxygen levels!

Jellyfish are opportunistic feeders. When food, like zooplankton, is abundant, they will grow rapidly and reproduce at a rapid rate. Excessively large jellyfish populations can out compete young fishes that also feed on zooplankton.

Some scientists speculate that the reproduction of jellyfish predators may also be giving jelly populations a boost. For example, all seven species of sea turtle will opportunistically feed on jellyfish when they are encountered. The largest species of sea turtle, the leatherback, feeds almost exclusively on jellies. Pacific populations of leatherbacks are currently at about 7 percent of their historic population levels. Human activities are the cause of sea turtle population declines. Overfishing has reduced fish populations that also feed on jellies.

To listen the full “Environmental Outlook” segment on Diane Rehm’s show, click here.